What is sender reputation?
Sender reputation is a mailbox provider’s running score of how much your mail is wanted. It is not one number: your domain and your sending IP are scored separately, from different inputs, and on a shared provider IP you control the domain score and almost none of the IP one.
Written by the SieveGuard team from the checks our own free tools run. Last reviewed .
Two scores, not one
Almost every deliverability conversation collapses these into "reputation", which makes the advice that follows unusable. They have different inputs, different timescales, and crucially different owners.
| Domain reputation | IP reputation | |
|---|---|---|
| Attached to | The domain in the From: address | The sending server’s address |
| Who owns it on a shared ESP IP | You, entirely | Your ESP and every other customer |
| Moves | Slowly — weeks to build, days to lose | Faster, and can swing on a neighbour |
| Survives changing ESP | Yes — it follows you | No — you inherit a new one |
| Fix time | Weeks of good sending | Days, or immediately by moving |
The row that matters is the last-but-one. Domain reputation follows you; IP reputation does not. So migrating ESPs fixes an IP problem and does nothing at all for a domain problem — which is why the migration so often produces no improvement, and why diagnosing which half is broken has to come first.
What actually feeds the score
Providers do not publish weightings, and anyone quoting exact percentages is guessing. The ordering below is well-established from provider documentation and postmaster tooling; the numbers are not, so there are none.
| Signal | Direction | Weight |
|---|---|---|
| Spam complaints ("report spam") | Down, hard | Heaviest single input |
| Hitting spam traps | Down, hard | Severe — a pristine trap is close to conclusive |
| Hard bounces | Down | Heavy above roughly 2% |
| Recipients rescuing mail from spam | Up | Strong positive, hard to engineer |
| Replies | Up | Strong positive |
| Opens and clicks | Up | Moderate — and noisy since image proxying |
| Deleting without reading | Down | Mild but cumulative |
| Volume consistency | Up | Steady beats spiky, always |
| Authentication passing | Gate | Not a boost. Failing costs you |
Gmail states a complaint-rate ceiling of 0.3%. That is roughly three complaints per thousand delivered messages — a much smaller budget than most senders assume, and the fastest way to lose a domain score you spent months building.
Note the last row. Passing SPF, DKIM and DMARC earns you nothing — it is the entry condition. Failing them is what costs.
Diagnosing which half is broken
Symptoms of a domain problem
Placement is bad across every provider and does not change when you send through a different service. Reputation in Google Postmaster Tools reads low or bad for the domain while the IP reads fine. Mail from a brand-new domain on the same infrastructure delivers normally.
Symptoms of an IP problem
Placement is bad from one sending service and fine from another with the same From: domain and the same content. Or it degraded suddenly with no change on your side, which is the signature of a neighbour on a shared IP — worth a blacklist check at that point.
The test that separates them
Send identical content from the same domain through two different services. Same result from both means the domain. Different results mean the IP. It takes an afternoon and it saves you from migrating for no reason.
Shared versus dedicated IP
The choice is almost entirely a function of volume, and the usual instinct is backwards.
| Shared IP | Dedicated IP | |
|---|---|---|
| Right for | Under roughly 100k messages a month | Sustained high volume |
| Reputation | Pooled — you inherit the pool’s | Entirely yours, good and bad |
| Starting from cold | Not an issue — the pool has history | Zero history. Must be warmed |
| Bad neighbour risk | Real, and outside your control | None |
| Volume gaps | Harmless | Damaging — an idle IP goes cold |
A dedicated IP at low volume is worse than a shared one. Providers need consistent volume to form an opinion, and an IP sending a few thousand messages a month never accumulates enough signal — so it is permanently treated as an unknown. The pooled reputation of a reputable ESP beats an unknown IP most of the time.
Building it, and rebuilding it
There is no fast path. Reputation is a moving average over weeks, so the only lever is consistent behaviour over time.
- Stop the bleeding first. Suppress anyone who has not engaged in six months. Counter-intuitive, and it works: complaint rate is a ratio, and the fastest way to improve it is to stop mailing the people generating complaints.
- Send to your most engaged recipients only for two to four weeks. You are re-teaching the filter what your mail looks like when it is wanted.
- Keep volume steady. A predictable daily pattern is read as legitimate; a spike after silence is read as a compromised account.
- Widen slowly, in engagement order, watching complaints at each step.
- Never buy a list while recovering, or at all. One send to purchased data undoes months of this.
On a genuinely new domain the same sequence is called warmup, and the difference is only that you are building from nothing instead of digging out.
Where to actually see it
Google Postmaster Tools is the only first-party view worth having, and it is free: domain and IP reputation, complaint rate, authentication results and delivery errors, for Gmail. It requires verifying the domain by DNS and needs a few hundred messages a day before it shows anything.
Microsoft offers SNDS and JMRP, both IP-based, both useful if you run your own infrastructure. Everything else is a third-party estimate — sometimes indicative, never the score itself. The providers do not publish it, and a vendor claiming to report your reputation score as a number is reporting their own model.
Sources
FAQ
What is the difference between domain and IP reputation?+
Domain reputation attaches to the domain in your From: address and follows you everywhere, including to a new provider. IP reputation attaches to the sending server and is shared with every other customer on that address. On a shared ESP IP you control the first and almost none of the second.
How long does it take to build sender reputation?+
Weeks of consistent sending to engaged recipients. It is a moving average, so there is no way to accelerate it — and it drops far faster than it climbs. A single send to a bad list can undo a month.
Should I get a dedicated IP?+
Only at sustained high volume, roughly 100,000 messages a month upward. Below that a dedicated IP never accumulates enough signal for providers to form an opinion, so it stays an unknown quantity — which is worse than inheriting a reputable ESP’s pooled reputation.
What is an acceptable complaint rate?+
Gmail states 0.3% as the ceiling, which is about three complaints per thousand delivered messages. That is a smaller budget than most senders expect, and exceeding it is the fastest way to lose a domain score built over months.
Where can I actually see my reputation?+
Google Postmaster Tools for Gmail, and Microsoft SNDS for Outlook if you run your own infrastructure. Both are free and first-party. Everything else is a third-party estimate — useful as a signal, but not the score the provider is actually using.
I switched ESPs and nothing improved. Why?+
Because the problem was your domain, not your IP. Changing provider gives you a new IP reputation and carries your domain reputation across unchanged. Run the same content from two services before migrating — identical results point at the domain.
Check it on a real domain
Free, no signup, DNS-only. Nothing you type leaves the lookup.